William Hague has intensified a Tory campaign against Tony Blair by warning the EU that his appointment as president of the European Council would be seen in Britain as a hostile act.

The shadow foreign secretary, who has said in private that Blair will only be appointed "over my dead body", told the 26 EU ambassadors in London that they should think carefully before choosing the former prime minister.

Blair is seen as the frontrunner to become the first permanent president of the European Council if the Lisbon treaty is ratified by all 27 EU member states.

European leaders are hoping to finalise a deal at their October summit in Brussels next week by persuading the Czech president Václav Klaus – leader of the only country not to have ratified the treaty – to accept it. A Czech signature would allow EU leaders to appoint to two new jobs under the treaty: a president of the European Council, who would ensure greater continuity between the rotating six-month EU presidencies; and a high representative for foreign affairs.

Hague, who last night met the US secretary of state, Hillary Clinton, amid concerns in Washington at the Tories' Euroscepticism, launched his strongest attack on Blair at a lunch for 26 ambassadors from the EU member states in London last Thursday. Eyebrows were raised when Hague said:

• British voters, who are about to remove a Labour government, would regard his reappearance on the world stage as a hostile act.

• Relations between a Conservative government and the EU would be worsened if Blair were president.

• Blair had favoured the US over the EU during his decade as prime minister.

• EU countries should remember the Iraq war and what that had done to European unity. Iraq would haunt Blair in his first few months as president because he would have to give evidence in public to the Chilcot enquiry into the 2003 war.

One source said: "It would be fair to say the ambassadors were taken aback. They know there are differences between Labour and the Conservatives but they were surprised that William Hague could not see the advantages for Britain in having such a big European post."

Tory sources say the party leadership is adopting a hard cop, soft cop approach to the appointment of Blair. Hague will lead the charge against the former prime minister while David Cameron will keep his powder dry in case he has to deal with a President Blair as prime minister.

One Tory source said: "Do not underestimate the strength of William's feelings about Tony Blair. William was badly hit by his loss to Blair in the 2001 general election. He also regards Blair as amoral in light of his handling of the Iraq war."